The present invention relates to an operational amplifier, and more particularly, to an operational amplifier for canceling an offset from an output signal.
In recent years, an LSI incorporates a large number of operational amplifiers. An operational amplifier is a basic circuit mounted on an LSI. However, the output signal of the operational amplifier contains errors caused by characteristic variations in the transistors configuring the operational amplifier. Therefore, errors must be cancelled from the output signal through a simple method. Further, an output signal of the operational amplifier is used to monitor the output signal at any given point of time. Thus, an operational amplifier is required to continuously generate an output signal.
In the prior art, many operational amplifiers are used to amplify analog signals or amplify differential signals. However, since the output signal of an operational amplifier contains errors, the output signal is not 0 V even if an input signal is 0 V. Such an error in the output signal with respect to the input signal is referred to as an offset voltage. In FIG. 1, the offset voltage is represented by a voltage source 2, which is connected to an input terminal of the operational amplifier 1.
The output voltage Vo of the operational amplifier 1, which is determined by an input voltage Vin, an input resistance R1, and a feedback resistance R2, is obtained from the equation shown below.Vo=(1+R2/R2)×Vin 
However, the offset voltage e1, which corresponds to the voltage source 2 shown in FIG. 1, is actually superimposed on the input voltage Vin. The output voltage Vo is thus obtained from the equation shown below using the offset voltage e1.Vo=(1+R2/R2)×(Vin−e1)
In other words, the offset voltage e1 is amplified together with the input voltage Vin. The offset voltage e1 contained in the output voltage Vo is thus large when the input voltage Vin is small.
To cancel the offset voltage of the operational amplifier, the area of the elements configuring the operational amplifier may be increased. When the element area is increased by two times, this method generally reduces the output error (offset) to the square root of ½. However, elements with large areas are needed to minimize the offset. This increases costs.
A method proposed to cancel the offset voltage of the operational amplifier without having to increase element area includes short-circuiting the two input terminals of the operational amplifier, holding the output voltage of the operational amplifier as the offset voltage, and feeding back the held voltage to the operational amplifier.
For example, Japanese Laid-Open Patent Publication No. 8-18353 discloses an operational amplifier including a main amplifier, an auxiliary amplifier, and a holding means. The auxiliary amplifier receives the output signal of the main amplifier and operationally amplifies the output signal in a reverse direction of the main amplifier. The holding means holds the output signal of the auxiliary amplifier and feeds back the held voltage to the main amplifier. The operational amplifier reduces the offset error of the main amplifier to an inverse of the gain of the main amplifier.
Japanese Laid-Open Patent Publication No. 2001-292041 discloses another example of an operational amplifier. The operational amplifier accumulates voltage, which is amplified by an operational amplification circuit and contains an offset, in a capacitor. The voltage value of the operational amplifier is feedback controlled based on the accumulated voltage.